COOLING PERFORMANCE OF A WET ROOFPOND SYSTEM IN LAS VEGAS, NEVADA.

Transcription

1 COOLING PERFORMANCE OF A WET ROOFPOND SYSTEM IN LAS VEGAS, NEVADA. Alfredo Fernández-González Florin Iulian Costache University of Nevada, Las Vegas 4505 Maryland Parkway, Box Las Vegas, NV alfredo.fernandez@unlv.edu costache@unlv.nevada.edu ABSTRACT This article compares and evaluates the cooling performance of wet and dry roofponds constructed at the Natural Energies Advanced Technologies (NEAT) Laboratory of the University of Nevada, Las Vegas. The experimental setup consisted of two identical test cells, each featuring a 6-inch deep roofpond with movable insulation. The only difference between the two roofpond test cells was that one had a dry roofpond while the other one had its roofpond water-sprayed at night in order to take advantage of indirect evaporative cooling. The indoor and outdoor environmental conditions presented in this article were experimentally measured during the astronomic summer (June 21 st through September 21 st ) of The measured data shows that under typical summer conditions, a dry roofpond is able to keep the maximum indoor operative temperature approximately 12 F below the maximum outdoor air temperature, with the minimum indoor operative temperature remaining approximately 5 F above the minimum outdoor air temperature. A wet roofpond is able to improve the performance of the dry roofpond, by lowering the maximum indoor operative temperature an additional 8 F (for a total reduction of approximately 20 F ), while also maintaining the minimum indoor operative temperature approximately 4 F below the minimum outdoor air temperature. By changing the ceiling to wall surface ratio of the test cells, and assuming that external walls are shaded in a way that produces the same indoor surface temperature profile measured in the north wall, the maximum indoor operative temperature of a wet roofpond building could be lowered an additional 3 F below the experimentally measured values. 1. INTRODUCTION Although passive solar heating and cooling strategies have been widely used throughout history, research on passive cooling has only been conducted during the last five decades (Givoni, 1994). During the 1960s, the cooling and heating potential of various roofpond configurations was investigated by Harold Hay and John Yellott (Hay &Yellott, 1968). The Skytherm TM system, invented and patented by Harold Hay, was first used on a 1,192 ft 2 single family residence in Atascadero, California (Haggard et al., 1975). This residence was followed by several other Skytherm TM prototypes throughout the United States (Marlatt, Murray, & Squier, 1984). In spite of the documented benefits, successful performance and significant energy savings produced by roofponds, the vast majority of buildings in the United States still continues to use mechanical systems for the heating and cooling of indoor spaces (Fernández- González, 2007). This article evaluates the cooling potential of a wet roofpond installed on a test cell during the summer of 2011 in Las Vegas, NV. Previous research at the NEAT Laboratory demonstrated that a 9-inch deep dry roofpond could reduce the daily maximum indoor operative temperature by approximately 12 F below the daily maximum outdoor air temperature (Fernández-González and Hossain, 2010). The experiment reported in this article builds upon previous research and compares two identical test cells, each featuring a 6-inch deep roofpond with movable insulation, where the only difference between them was that one of the test cells had a dry roofpond while the other one had its roofpond water-sprayed at night in order to take advantage of indirect evaporative cooling. 1

2 2. ROOFPOND BASICS The Skytherm TM system is a passive strategy that can provide both heating and cooling. In its heating mode, the roofpond is exposed to the sun s radiation during the day, allowing for the roofpond s thermal storage to collect the sun s thermal energy. At night, the roofpond is insulated and sealed off from the outdoor conditions in order to minimize heat losses to the environment. separates the structural ceiling from the contained water mass is an EPDM liner. Thermacore automated garage doors provide the movable insulation for the roofponds. The R-value of the movable insulation used in this experiment is 11 h-ft 2 F /Btu (1.94 m 2 C / W). In its cooling mode, which is the focus of this paper, the process is reversed: the thermal storage is covered with the movable insulation during the daytime in order to minimize heat absorption; at night, the roofpond is exposed to the night sky in order to dissipate the heat absorbed during the daytime into the atmosphere (see Fig. 1). As a result, the roofpond is able to provide radiant cooling to the space below during both daytime and nighttime. Day Fig. 1: Skytherm TM system in cooling mode (Source: Yannas, Erell, & Molina, 2006, 26) 3. EXPERIMENTAL SETUP Night As stated earlier, previous research has shown that, when operated correctly, a roofpond can reduce the daily maximum indoor operative temperature by approximately 12 F below the daily maximum outdoor air temperature. However, in Las Vegas, the daily maximum outdoor temperature can easily reach over 110 F during the summertime. As a result, the indoor operative temperature achieved by a dry roofpond would still be above a comfortable level. Taking advantage of Las Vegas low relative humidity, a new experiment was setup at the NEAT Laboratory to explore the potential of a roofpond assisted with indirect evaporative cooling Description of the experimental set-up Both test cells have interior dimensions of 4-3 x 6-10 x 8-0 (see floor plan in Fig. 2). A corrugated metal deck is used as structural ceiling. A 6-inch deep water mass contained within sealed polyethylene bags sits on top of this structural ceiling (see section in Fig. 2). The only thing that Fig. 2: Plan and section of roofpond test cells (Source: Fernandez-Gonzalez and Kako, 2010, 2) 3.2. Operation of the test cells The temperature readings for this experiment were taken during the summer of 2011, starting on June 12 th and ending on September 30 th. Between June 12 th and August 20 th, the movable insulation panels covered the roofponds every morning starting at 6:00 AM. The movable insulation panels of both test cells remained closed for 14 hours to reduce heat gains from incident solar radiation. The movable insulation panels were retracted every evening at 8:00 PM, when the environment was cooler and could begin to absorb the heat gained by the roofpond throughout the day. On August 21 st the cycle was changed to cover the roofponds at 6:30AM and retract the movable insulation at 7:30 PM. This way, the roofponds were able to take advantage of one additional hour to dissipate the absorbed heat into the environment. Starting on September 11 th, the cycle was modified once again to close the movable insulation at 7:00AM and open it after 12 hours, at 7:00PM. Both test cells featured an exhaust fan that was operated hourly to maintain a ventilation rate of 1 ACH during these experiments. 2

3 Starting on June 18 th and until the end of the experiments, water was pumped from an adjacent storage tank and delivered to the upper surface of one of the two roofponds using a drip irrigation system (see Figs. 3 and 4). The operation of the wet roofpond (W-RP) required spraying water every evening to augment its cooling capacity by taking advantage of the indirect evaporative cooling potential that results from hot and dry clear nights. At the beginning of the experiment, 10 gallons of water were delivered per night to the wet roofpond. However, after careful observation, it was noted that a maximum of 3 gallons per night (which corresponds to approximately 1 gal. / 10 ft 2 of roofpond surface area) could be successfully evaporated during the hottest part of the summer. Therefore, the water delivery rate for the wet roofpond was established at 3 gallons per night until the end of the experiment. Fig. 5: Dry Roofpond (D-RP) (Photo credit: Florin Iulian Costache) 3.3. Instrumentation The equipment used to monitor the test cells was first tested and calibrated in a controlled environment in order to ensure accurate measurements. Temperature sensors were placed inside and outside both cells as follow: Fig. 3: NEAT Lab test cells with water tank used for irrigating the W-RP (Photo credit: Florin Iulian Costache) Indoor sensors: A total of eight thermistors (one for the ceiling, one for the floor, and one for each vertical surface orientation) were installed inside each of the roofpond test cells to measure their respective indoor surface temperatures. These thermistors were connected in pairs to four HOBO H-8 RH/Temperature 2x data loggers. A fifth HOBO H-8 RH/Temperature 2x data logger was added to each test cell in order to measure the indoor relative humidity, the Black Globe Temperature, and the indoor air temperature at 3.6 ft. (1.1 m.) above the finished floor (see Fig. 6). Outdoor sensors: On the exterior, both test cells featured thermistors that recorded the surface temperature of all vertical surfaces, the top and bottom faces of the movable insulation, and the top and bottom surfaces of the EPDM liner. The thermistor placed above the EPDM liner is used as a proxy to describe the temperature of the thermal storage. All these thermistors were connected to a total of six (three per test cell) HOBO U-12 Outdoor Industrial data loggers. 4. RESULTS Fig. 4: Wet Roofpond (W-RP) (Photo credit: Florin Iulian Costache) The data experimentally acquired during the summer of 2011 was analyzed and summarized to illustrate the hourly average performance of dry and wet roofponds throughout the entire summer (June 21 st through September 21 st ), the 3

4 performance during an early-summer monsoon day (July 4 th ), and the performance during the day in which the outdoor air temperature reached its highest value (August 24 th ). These results provide a clear indication of the great potential offered by roofponds as a cooling strategy for the U.S. Southwest. A comparison between the average minimum outdoor air temperature (81.2 F) and the average minimum indoor operative temperature measured within the dry roofpond (84.5 F), demonstrates that without the assistance of nighttime evaporation, minimum indoor operative temperatures remain slightly above (3.3 F ) the outdoor minimum air temperature. However, once nighttime evaporation is introduced, the indoor operative temperature of a wet roofpond remains practically at all times below the outdoor air temperature, achieving a reduction of its average minimum operative temperature of almost 4 F below the average minimum air temperature (see Fig. 7) Performance during a rainy (monsoon) day At the beginning of the summer, Las Vegas, like most other cities in the US Southwest, experiences isolated rain events during a relatively brief monsoon season. To exemplify the performance of dry and wet roofponds under such conditions, the hourly temperatures recorded on July 4 th, 2011 are presented in Fig. 8. It is worth mentioning that the evening of July 3 rd was unusually wet, setting a precipitation record for that day with a measured rainfall of 0.74 inches (this amount is equivalent to approximately 17% of the annual rainfall for Las Vegas). In such a situation, the dry and wet roofponds benefited from the rain and had some degree of indirect evaporative cooling due to the rain that accumulated on their respective roofs. The night between July 3 rd and July 4 th was also the only time in which the wet roofpond s indoor operative temperature remained slightly above the outdoor air temperature (see Fig. 8). Due to its thermal inertia, the wet roofpond s indoor operative temperature remained below the dry roofpond s indoor operative temperature (see Fig. 8). Fig. 6: Instrumentation of NEAT Lab test cells (Photo credit: Florin Iulian Costache) 4.1. Overall summer performance The experimental results obtained during the summer of 2011 confirmed that both roofpond types are able to provide remarkable indoor thermal stability. While the average outdoor air diurnal temperature swing during the period was 20.4 F, the average indoor operative temperature swing within both roofponds was below 9 F. A comparison between the average maximum outdoor air temperature (101.6 F) and the average maximum indoor operative temperature measured within the dry roofpond (92.7 F) shows a reduction of 8.9 F. The nightly evaporation of approximately 3 gallons of water helped further reduce the average maximum indoor operative temperature within the wet roofpond to 86.2 F, for a reduction of 15.4 F below the average maximum outdoor air temperature (see Fig. 7) Performance during the hottest summer day In a typical Las Vegas summer, the daily maximum outdoor air temperature continues to rise until it reaches its highest values during the month of August. It is during this part of the summer that the wet roofpond delivers its best performance. Fig. 9 illustrates the behavior of dry and wet roofponds during August 24 th, 2011, which was the day with the highest outdoor air temperature during the period discussed in this article. A comparison between the daily maximum outdoor air temperature (111.8 F) and the daily maximum indoor operative temperature measured within the dry roofpond (100.3 F) shows a temperature reduction of 11.5 F. While this reduction is significant, the operative temperature measured inside the dry roofpond remains outside the comfort zone during the entire day (see Fig. 9). The nightly evaporation of approximately 3 gallons of water helped reduce the daily maximum indoor operative temperature 4

5 within the wet roofpond to 91.3 F, for a temperature reduction of 20.5 F below the daily maximum outdoor air temperature (see Fig. 9). This is a highly significant reduction that almost brings the wet roofpond s daily maximum indoor operative temperature (91.3 F) to be within the upper 80% acceptability limits established by the ASHRAE Adaptive Model for Comfort (which would be 88 F for Las Vegas hottest month of the year). Fig. 7: Average hourly summer temperatures (June 21 st through September 21 st, 2011) A comparison between the daily minimum outdoor air temperature (84.5 F) and the daily minimum indoor operative temperature measured within the dry roofpond (90.2 F) during the hottest day of the summer shows that the dry roofpond s minimum indoor operative temperature remains 5.7 F above the daily minimum outdoor air temperature. The minimum indoor operative temperature measured within the wet roofpond during the hottest day of the summer was 80.6 F, remaining almost 4 F below the daily minimum outdoor air temperature (see Fig. 9). 5. IMPLICATIONS OF THE RESULTS Fig. 8: Hourly temperatures - July 4 th, 2011 (monsoon day) A closer look at Figures 7 through 9 shows that the indoor operative temperatures measured within the wet roofpond were for the most part below 88 F, which constitutes the upper threshold of the 80% acceptability zone established by the ASHRAE Adaptive Model for Thermal Comfort. Furthermore, a closer examination of the ceiling surface temperatures measured inside the wet roofpond illustrates how this strategy successfully cools the ceiling to the point in which it can absorb heat from within the space at all times, while remaining as an effective source for radiant cooling even during the hottest day of the summer (Fig. 9). These observations prompted two hypotheses that are discussed in the remaining sections of this paper Changing the ceiling to wall surface ratio Evidently, the ceiling s surface temperature is the driving force behind the cooler operative temperatures experienced inside both roofponds. Given the unique ceiling to wall surface (C/W) ratio that exists in the experimental test cells, it would be interesting to find out what might happen when a wet roofpond is deployed over a hypothetical model having a more typical C/W ratio. Fig. 9: Hourly temperatures - August 24 th, 2011 (hottest summer day) Fig. 10: Dimensions of experimental test cell (left) and hypothetical model (right) 5

6 Assuming that a wet roofpond is installed over a building with dimensions of 20' x 40' x 8 (see Fig. 10), and using the measured indoor surface temperature data for the wet roofpond (W-RP), one can calculate the resulting Mean Radiant Temperature (MRT) for this new building. MRT Calculation: Fig. 2 illustrates the NEAT Laboratory test cells interior dimensions, which are: 4'-3" x 6'-10" x 8'. These dimensions produce a ceiling to wall surface ratio: C/W NEAT = 0.16 The hypothetical model s dimensions considered in this comparison are 20' x 40' x 8'. These dimensions produce a ceiling to wall surface ratio: C/W MODEL = 0.83 Using the ISO 7726 (1985) formula to calculate the Mean Radiant Temperature, we have: MRT = Σ (F i T i ) (1) where: T i = the temperature of the surface. F i = the solid angle factor between a point and a surface, which could be calculated as: F i = Ω i / 4π (2) where: Ω i = the solid angle described by the interior surfaces of the building to the point where the MRT is calculated. The value of the solid angle (Ω) in a rectangular pyramid could be calculated with the formula: Ω = 4tan -1 [ ab / [2d (4d 2 +a 2 + b 2 ) 0.5 ]] (3) where: a = length of the rectangular base of the pyramid. b = width of the rectangular base of the pyramid. d = distance between the center of the base and the apex of the pyramid. Tables 1 and 2 present the calculated solid angle factors (F) for the NEAT Laboratory test cells and the hypothetical model, respectively. Using the calculated solid angle factors, it is then possible to calculate the MRT for the hypothetical model assuming that the surface temperatures of the indoor ceiling, floor, and walls are exactly the same as those measured inside the NEAT Laboratory wet roofpond test cell. It seems appropriate to mention that the air temperature and the MRT inside the roofpond test cells were at all times within 1 F during the monitored period. TABLE 1: SOLID ANGLE FACTORS FOR TEST CELLS A b d Ω F Floor (f) Ceiling (c) North (n) East (e) South (s) West (w) Substituting the solid angle factors in Equation 1, the calculation of the MRT for the NEAT Laboratory roofpond test cells becomes: MRT = 0.12T f T c T n T e T s T w where T f, T c, T n, T e, T s, and T w are the hourly measured indoor surface temperatures of the floor, ceiling, north, east, south, and west walls, respectively. TABLE 2: SOLID ANGLE FACTORS FOR MODEL a b d Ω F Floor (f) Ceiling (c) North (n) East (e) South (s) West (w) Similarly, by substituting the hypothetical model s solid angle factors in Equation 1, the calculation of the MRT for the hypothetical model becomes: MRT = 0.38T f T c T n T e T s T w where T f, T c, T n, T e, T s, and T w are the same hourly measured indoor surface temperatures used in the calculation of the MRT of the wet roofpond test cell. The calculated MRT for the wet roofpond test cell (W-RP) and the hypothetical model (MODEL) are presented for the same three periods discussed in the results section of this paper, namely, an average summer day (Fig. 11), July 4 th, which represents a monsoon day (Fig. 12), and August 24 th, which was the hottest summer day during the experimental period covered in this article (Fig. 13). Overall Summer Performance of Hypothetical Model: A comparison between the average maximum indoor operative temperature within the wet roofpond (86.2 F) and the calculated maximum MRT for the hypothetical model (84.6 F) demonstrates that the benefits offered by this passive cooling strategy are likely to increase if it is deployed in buildings with a higher C/W ratio (Fig. 11). 6

7 A reduction of less than 1 F was also appreciated between the wet roofpond s measured average minimum indoor operative temperature (77.3 F) and the calculated minimum MRT for the hypothetical model (76.7 F). Performance during July 4 th (monsoon day): A smaller reduction between the wet roofpond s measured daily maximum operative temperature (86.5 F) and the calculated maximum MRT for the hypothetical model (85.7 F) was found during a monsoon day (Fig. 12). The calculated minimum MRT of the hypothetical model was inconsequentially lower than the wet roofpond s measured daily minimum operative temperature (Fig. 12). Performance during August 24 th (hottest summer day): During the hottest day, the measured maximum operative temperature within the wet roofpond was 91.3 F, while the maximum calculated MRT for the hypothetical model was 89.5 F (Fig. 13). This further demonstrates that wet roofponds work best during the hottest (and therefore driest) part of the summer in Las Vegas. A reduction of approximately 0.5 F was also appreciated between the wet roofpond s measured minimum operative temperature (80.6 F) and the calculated minimum MRT for the hypothetical model (80 F). Fig. 11: Average hourly measured and calculated summer temperatures (June 21 st through September 21 st, 2011) 5.2. External shading and improved wall construction The second hypothesis that is explored in this article has to do with the siting of the NEAT Laboratory test cells and the fact that the east wall is primarily an access door with an overall R-value of 3 h-ft 2 F /Btu (0.529 m 2 C / W). With respect to the siting of the NEAT Laboratory test cells, the experimental results suggest that when walls are externally shaded (as is the case of the north and south walls in both test cells), their indoor surface temperature experiences a noticeable reduction when compared to walls that are exposed to direct solar radiation (as is the case of the east and west walls in both test cells). To that end, it would be interesting to see what would happen to the indoor operative temperature of the hypothetical model if all four of its walls had the same indoor surface temperature profile. To explore this idea, the hourly measured indoor surface temperature of the wet roofpond north wall was selected and applied to all four walls to recalculate the MRT of the hypothetical model. To that end, the calculation of the MRT for the hypothetical model now becomes: Fig. 12: Measured and calculated hourly temperatures - July 4 th, 2011 (monsoon day) MRT = 0.38T f T c T n The results from this speculation suggest that the MRT of the hypothetical model would be further decreased by approximately 1 F if all of its external walls were to be externally shaded. This temperature decrease is slightly more pronounced during the daytime. Fig. 13: Measured and calculated hourly temperatures - August 24 th, 2011 (hottest summer day) 7

8 It is worth mentioning that this assumption is not entirely unrealistic, as new residential buildings are oftentimes placed six feet apart and the two exposed facades could easily feature an overhang to shade both windows and walls (as it sometimes the case). 6. CONCLUSIONS The measured and calculated temperatures presented in this article demonstrate that a Skytherm TM roofpond coupled with indirect evaporative cooling could be a powerful strategy to provide the majority (if not all) of the space cooling needed by a building with low to moderate internal heat gains in Las Vegas, Nevada. During the hottest (and therefore driest) part of the summer, the wet roofpond exhibited its best performance, achieving a reduction of more than 20 F between the maximum outdoor air and indoor operative temperatures. This temperature reduction decreases during periods of rain and/or high humidity. Similarly, the minimum operative temperature of a roofpond is able to remain approximately 4 F below the minimum outdoor air temperature, providing significantly cool ceiling temperatures that can absorb heat from within the space at all times while remaining an effective source for radiant cooling, even during the hottest part of the summer. Simple design and/or construction improvements discussed in this article suggest that it may be possible to further decrease the daily maximum indoor operative temperature of a wet roofpond building by an approximate 3 F. Further reductions may be obtained by reducing infiltration from 1 ACH to 0.6 ACH, by coupling the building to the ground, and by increasing the R-value of building envelope components. REFERENCES (1) Givoni, B., Passive and Low Energy Cooling of Buildings. New York, NY, Van Nostrand Reinhold, 1994 (2) Hay, H., Yellott, J., International Aspects of Air Conditioning with Movable Insulation. Solar Energy, 12, , 1968 (3) Haggard, K., Hay, H., Saveker, D., Edmiston. J., Feldman, J., Hawes, M., et al, Research Evaluation of a System of Natural Air Conditioning. California Polytechnic State University. San Luis Obispo: California Polytechnic State University, 1975 (4) Marlatt, W. P., Murray, K. A., Squier, S. E., Roofpond Systems. U.S. Department of Energy, Energy Technology Center, Canoga Park, CA: Energy Systems Group, Rockwell International, 1984 (5) Fernández-González, A., Analysis of the Thermal Performance and Comfort Conditions Produced by Five Different Passive Solar Heating Strategies in the United States Midwest. Solar Energy, 81, , 2007 (6) Fernández-González, A., Hossain, A., Cooling Performance and Energy Savings Produced By a Roofpond in the United States Southwest. American Solar Energy Society, 2010 (7) Yannas, S., Erell, E., Molina, J. L., (2006). Roof Cooling Techniques - A Design Handbook. Earthscan: London, UK, 2006 (8) Fernández-González,A., Kako, I. K., Empirically Derived Formulas to Predict Indoor Maximum, Average, and Minimum Temperatures in Roofpond Buildings Using Minimum Climatic Information. American Solar Energy Society, 2010 (9) ISO 7726, Thermal Environments Instruments and Methods for Measuring Physical Quantities. International Organization for Standardization: Geneva, Switzerland, 1985 (10) Pahikkala, J., "solid angle" (version 22). PlanetMath.org, available at 8